1. Field of the Invention
The present invention relates to a method of reproducing the phase-change waveform and intensity waveform of optical short-pulse in the time domain that combines measurement of the autocorrelation waveform of optical short-pulse using two photoconductors with field power spectrum measurement using an optical spectrum analyzer.
2. Description of the Related Art
Developments in the field of ultra-high-speed optical communication have increased the need for the measurement of the intensity waveform of optical short-pulse as well as the measurement of change in chirp frequency over time. The conventional method of measuring the intensity waveform of optical short-pulse involves the use of two photoconductors.
FIG. 1 shows a prior-art measurement system of an autocorrelation waveform employing photoconductors. Explanation of this measurement method will refer to this figure.
The generation source 402 of optical short-pulse that is to be measured generates a line of optical short-pulse to be measured at the timing of the rise or fall of output signals generated by signal generation source 401. This line of optical short-pulse that is to be measured is modulated by passage through optical ON/OFF shutter 404 which turns ON and OFF in synchronism with low-frequency (several 10 Hz to several 100 Hz) pulse signals generated at function generator 403.
The line of optical short-pulse that is to be measured following modulation by passage through optical ON/OFF shutter 404 is divided into two branches by half-mirror 405, one branch being irradiated into first photoconductor 408 directly (or by way of a fixed mirror), and the other branch being irradiated into second photoconductor 409 by way of movable mirror 407.
First photoconductor 408 and second photoconductor 409 enter a conductive state if irradiated by light when in a state in which a phase difference occurs between both ends. As shown in the figure, second photoconductor 409 enters a conductive state only when light is irradiated onto first photoconductor 408, and as a result, an autocorrelation waveform can be obtained by conferring delay times to each photoconductor and irradiating optical pulses.
Movable mirror 407 is mounted on XY stage 406 which is driven by stage driver 412. By controlling stage driver 412 so as to shift XY stage 406 according to the state of the input to lock-in amplifier 411 of output signals arising at sampling output terminal 410 of second photoconductor 409, the value of a delay time .tau., which is the difference in signal input time to first photoconductor 408 and second photoconductor 409, can be gradually varied. Here, the pulse output of function generator 403 is inputted to lock-in amplifier 411 as a reference signal.
The output of lock-in amplifier 411 at this time is substantially equal to the autocorrelation waveform of the optical pulse intensity waveform, and this output is taken in by way of CPU 413 and displayed on data display 414.
The above-described measurement method measures the autocorrelation waveform and therefore is not influenced by jitter. As a result, measurement can theoretically be achieved at a resolution on the order of several femtoseconds by employing photoconductors of extremely high-speed response.
In measurement according to the above-described prior art, the object of measurement is nevertheless the autocorrelation waveform l(.tau.) of optical short-pulse, and therefore, what is found from this autocorrelation data is limited to the power spectrum .vertline.l(.omega.).vertline..sup.2 of the intensity waveform i(.tau.) of optical short-pulse. Accordingly, the measurement method of the above-described prior art is limited to finding only an approximation of the half-width of the optical short-pulse, and has the shortcomings of not enabling measurement of the intensity waveform itself of optical pulses, and of not enabling measurement of the intensity waveform of optical short-pulse or the change of chirp frequency over time.